This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
.☺., xdamian1000x, nicole.lely, clas622, gabgamers17, kamilo154, DatFreakingBoi, sehmuzbey63, -[FF]- klev, brandonsgalaxy04, lolka5907, iagoulisesromero05, jwright159, dioszs, O.W.N. medic, O.W.N. medic, Timo, Gorn, a72672094, a72672094, Don't be haedyn, Pepe, gamesop1122, capsa1560, diegodushan, jungar.hero, suebatey12, [TR]kafam_güzel, mossesx, gamerwanted7, emulatorslife, emulatorslife, emulatorslife, curmi16, MrKilZz, sanjabijelic41, avdik970, zee.ekspert, Backward arrows_Hungarian53, GojiPlaysGam3z, GOLDEN xD, Jason Voorhees, BeaconGamerZ, zom-_-bies, zsopyryaev, costin211.boss, xReemtix, Lord Demon, gorbatova-n-85, sabaidee2514 +402 others